Metabolic wastes or excretes are substances left over from respiratory processes, which cannot be used by the organism, and must therefore be excreted. This includes nitrogen compounds, water, CO2, phosphates, sulfates, indoles, medicals, food additives etc. Animals treat these compounds as excretes. Plants have chemical "machinery" which transforms some of them into useful substances, and it has been shown by Brian J. Ford that abscised leaves also carry wastes away from the parent plant. In this way, Ford argues that the shed leaf acts as an excretophore.